In a microwave assisted magnetic recording head, a spin torque oscillator (STO) is formed on the main magnetic pole or an auxiliary magnetic pole. The STO is formed to be electrically connected to the main magnetic pole and an auxiliary magnetic pole those are the electrodes to supply a current to STO. When a current is applied to pass to the STO, it flows through, for example, the main magnetic pole, an underlayer and a spin injection layer (SIL) in this order. As the current flows, the SIL causes a spin-torque effect on the main magnetic pole, thereby generating a spin wave in the main magnetic pole. Thus, conventional techniques entail such a drawback that the effective magnetic permeability of the main magnetic pole is reduced, degrading the recording characteristics.
Under these circumstances, there is a demand for suppression of generation of spin waves in the main magnetic pole against the spin-torque.